fix(desktop): standard Linux AppImage icons for AppImageLauncher

[mwolson]

What Changed

• Generate standard Linux hicolor icon sizes for AppImage builds from the selected desktop icon source.
• Point the Linux electron-builder config at the generated icon directory instead of a single source PNG.
• Install ImageMagick in the Linux release workflow when it is not already available.

Why

Closes #2331.

AppImageLauncher and Linux desktop shells can fail to show an integrated AppImage icon when the package only contains the large source icon. Shipping the usual hicolor app icon sizes gives desktop icon lookup a standard path such as usr/share/icons/hicolor/512x512/apps/t3code.png.

This keeps the source assets small and generates the resized Linux icons at package time instead of committing generated PNGs.

Related prior art: #838 looked at runtime AppImage icon lookup. This PR fixes the packaged AppImage icon-theme entries instead.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes: N/A, packaging-only change
• I included a video for animation/interaction changes: N/A

Validation

• bun fmt
• bun lint
• bun typecheck
• bun run test
• Built a local AppImage and verified AppImageLauncher integration shows the icon in Niri/Noctalia.
• Verified CI Linux AppImage artifacts for both channels:
  • Nightly: https://github.com/mwolson/t3code/actions/runs/24896075119
  • Release: https://github.com/mwolson/t3code/actions/runs/24897399770
• Extracted both AppImages and confirmed they include hicolor icon entries for 16, 22, 24, 32, 48, 64, 128, 256, and 512.
• Confirmed the nightly artifact uses the nightly icon art and the release artifact uses the release icon art.

---

Note

Medium Risk
Changes Linux packaging to generate and ship a multi-size icon theme and adds an ImageMagick build dependency, which could cause Linux release builds to fail or produce incorrect icons if tooling/config is off.

Overview
Linux AppImage packaging now generates a standard set of hicolor-sized app icons (16–512) during build-desktop-artifact, using ImageMagick (magick with convert fallback) and failing with a clear error if neither is available.

Electron-builder’s Linux config is switched from a single icon.png to the staged icons/ directory so the produced AppImage includes the multi-size icon set, and the release workflow installs ImageMagick on Linux runners when needed.

^{Reviewed by Cursor Bugbot for commit d5fce53. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Generate standard multi-size icons for Linux AppImage builds

• Adds stageLinuxIconSize in build-desktop-artifact.ts to resize the source PNG to standard Linux icon sizes (16–512px) using ImageMagick (magick or convert), with a clear error if neither is available.
• Updates the Electron builder icon config for Linux from icon.png to the generated icons directory so the packaged AppImage includes all sizes.
• Updates the release.yml CI workflow to install ImageMagick on Linux runners when neither magick nor convert is present.
• Risk: introduces a hard build-time dependency on ImageMagick; builds will fail with an explicit error if it is missing and cannot be auto-installed.

^{Macroscope summarized d5fce53.}

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[macroscopeapp]

Approvability

Verdict: Approved

Build-only changes that add Linux icon generation using ImageMagick during the desktop artifact build process. No runtime application code is modified - this only affects how AppImage packages are assembled with properly-sized icons.

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[mwolson]

Superseded by #2538.

While validating this under Niri, I found a second AppImage integration issue in the same flow: remote environment setup needed a launcher-level `--password-store=gnome-libsecret` workaround so Electron would use an encrypted Linux secret store.

I moved the AppImage icon fix from this PR into the new combined PR and added the secret-store fix there, with validation from a real Niri AppImage launch and a two-machine remote environment connection. Closing this narrower PR so review can happen in one place.